The present invention relates to a consumable cored wire comprising an optical fiber surrounded by a cover for measuring a temperature of a molten steel bath.
JPH0815040 (A) describes a method involving feeding a consumable optical fiber into liquid metal for measuring the temperature of molten metal baths, A similar method and apparatus for optical fiber measurements of molten metals is also described in U.S. Pat. No, 5,730,527. Consumable optical fibers of this kind are known, for example, from JPH111160155 (A). These early consumable optical fibers are single metal jacketed optical fibers in which the optical core is provided with a metal covering, typically stainless steel, that serves the purpose to stiffen the optical fiber such that it can be immersed into molten metal. Whereas these immersible optical fibers can penetrate beneath the molten surface, they also suffer from a rapid deterioration.
Improvements to these early consumable optical fibers comprise additional protection structures and are known, for example, from JH10176954 (A). In JPH10176954 (A), the optical fiber is surrounded by a protective metal tube which is surrounded by an additional layer of plastic material. This covered consumable optical fiber, winch is immersible into the molten metal, is fed from a coil or spool at a predetermined rate that would expose the tip of the optical fiber to the metal when deeply immersed. The depth of immersion at the time of exposure is important for temperature accuracy. Therefore, preventing early destruction or moving the optical fiber tip to the measuring point quickly are necessary for accurate temperatures. JPH09304185 (A) discloses a feeding rate solution where the speed of fiber consumption must be greater than the rate of devitrification, thereby assuring that a fresh optical fiber surface is always available.
Since heat from any source degrades the fiber, the fiber should be protected from heat gain just prior to immersion. Likewise, the remaining unused portion must also be protected from heat gain after a measurement to be suitable for the next measurement.
U.S. Pat. No. 5,585,914 teaches that a consumable optical fiber can be fed through a nozzle into a molten metal at a rate of 5 mm/sec for 10 seconds, then held as immersed for 20 seconds. When accomplished in a cyclical fashion, this process can be considered continuous. JPH09304185 (A) teaches that for accurate results, the rate of destruction and subsequent exposure of a new surface must correspond to the speed at which the vitreous structure of its tip is destroyed. That is, new fiber material is constantly fed to replace devitrified fiber and is thus suitable for receiving and passing on radiation, without radiation losses.
To accomplish this replacement, the fiber is fed into the molten metal until its temperature response exceeds a set point. The feeding is stopped for two seconds and a first temperature is determined. The fiber is then fed again into the metal for 10 mm and stopped for two seconds, and a second temperature is determined. A comparison of the first and seconds temperatures determines if a successful measurement has been achieved or if additional cycles are needed. Besides for a means to determine if the reading is acceptable, the speed of feeding is not specified.
Additionally, since steelmaking is a batch process, the above prior art suffers from the fact that a prior measurement using the remaining portion of coiled optical fiber will be devitrified. Therefore, one cannot follow this method since the initial determination of the set point temperature of the next measurement cannot be adequately determined. JPH09243459 (A) teaches a corrective action in that damaged consumable optical fibers should be cut away from the supply coil each time to provide an un-devitrified fiber. Yet, this prior art reference provides no indications of how one is to determine the extent of devitrification.
In practice, this requires additional equipment to cut away the damaged portion of the fiber and, in the case where the immersion is from above the metal, the fiber must be withdrawn through a layer of slag which may collect on the fiber, thus interfering with the removal from the vessel and the cutting mechanism.
A variety of schemes to feed consumable optical fiber all are designed to expose the optical fiber core to the molten metal before devitrification. However, the devitrification rate is dependent upon the actual conditions of the molten metal, such as its temperature, its motion, the containment vessel and the slag covering the bath, as well as the thermal conditions that the optical fiber is exposed to before and after each measurement cycle.
It has been found that since the availability of a fresh fiber surface is essential for an accurate temperature measurement and since this availability depends upon how the fiber is immersed into the molten metal, multiple feeding schemes are likely to arise due to the numerous variety of conditions that the fiber will be exposed to when immersed into and through various metallurgical vessels at various times during metals processing.
When variation in the rate of devitrification can be minimized by improvement in the consumable optical fiber construction, the applicability of the measuring technique can apply to a wider range of metallureical vessels without customization of the feeding regime.
Multi-layered wire structures with a steel outer covering are used in steelworks to introduce doping substances selectively into the molten steel bath. These are typically called cored wires and are described in DE19916235A1, DE3712619A1, DE19623194C1 and U.S. Pat. Nos. 6,770,366, 7,906,747 discloses a cored wire comprising a material which pyrolizes upon contact with a liquid metal bath.
U.S. Pat. No. 5,988,545 discloses a cored wire injection system where cored wires are supplied in coils or on spools, such as for integration with special wire feeding machines, such as disclosed in EP 0806640, JPH09101206 (A), JPS6052507 (A) and DE3707322 (C1), to carry out the practical immersion of the cored wire.
U.S. Pat. No. 7,748,896 discloses a device for measuring a parameter of a molten bath. The device comprises an optical fiber, a cover laterally surrounding the optical fiber, and a detector connected to the optical fiber, wherein the cover surrounds the optical fiber in a plurality of layers, one layer comprising a metal tube and an intermediate layer arranged beneath the metal tube, the intermediate layer comprising a powder or a fibrous or granular material, wherein the material of the intermediate layer surrounds the fiber in a plurality of pieces.
The intermediate layer is formed of silicon dioxide powder or aluminum oxide powder and may contain a gas producing material. The disclosed feature of the intermediate layer surrounding the fiber in a plurality of separate parts means, in the sense of the invention, that the construction in multiple parts exists in the operating state (i.e., during or after immersion in the molten bath to be measured), such that the pieces of the intermediate layer remain separate and are separable during use. The addition of a gas producing material aids in the explosive separation of the parts of the intermediate layer.
While the parts are contained within the un-melted outer metal jacket, this optical cored wire structure aids in keeping the optical fiber at its center at a very low temperature for a relatively long time. Denitrification from elevated temperatures that will destroy the optical fiber is delayed. From a particular temperature onwards during immersion into molten metal, expansion of the gases of the intermediate layer forcibly remove the un-attached cover layers.
The fiber is heated erratically to the equilibrium temperature in the molten metal bath, so that the measurement can then take place very quickly before the optical fiber or its end immersed in the molten metal bath is devitrified. This unpredictability of the explosive nature of the gas evolution to reveal a fresh optical surface, provides erratic results which are subject to interpretation and misreading of the correct temperature.
According to the known requirement of accurate temperature measurements by immersed optical fiber, the optical fiber must be consumed at a rate which is equal to or faster than the rate of devitrification of the optical core. Since the rate of devitrification is both a function of the amount of heat input to the optical core during its molten metal immersion and heat input to the optical core from exposure to the environment surrounding the cored optical fiber, the fiber must be allowed to be consumed in proportion to pre-exposure conditions, such as the radiant heat at the immersion location, the slag temperature as well as the melt temperature of the particular furnace.
An objective of the present invention is to further improve the measuring of the temperature in a molten steel bath by an improved cored wire.